[effects/maximize] Enable blur behind windows
Summary: Use the blur effect even while the window is being maximized/unmaximized.
Test Plan:
Before:
After:
Reviewers: KWin, VDG, graesslin, ngraham
Reviewed By: KWin, VDG, graesslin, ngraham
Subscribers: ngraham, graesslin, zzag, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D12623